LeetCode – Combination Sum III (Java)
Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.
Ensure that numbers within the set are sorted in ascending order.
Example 1: Input: k = 3, n = 7 Output: [[1,2,4]]
Example 2: Input: k = 3, n = 9 Output: [[1,2,6], [1,3,5], [2,3,4]]
Analysis
Related problems: Combination Sum, Combination Sum II.
Java Solution
public List<List<Integer>> combinationSum3(int k, int n) { List<List<Integer>> result = new ArrayList<List<Integer>>(); List<Integer> curr = new ArrayList<Integer>(); helper(result, curr, k, 1, n); return result; } public void helper(List<List<Integer>> result, List<Integer> curr, int k, int start, int sum){ if(sum<0){ return; } if(sum==0 && curr.size()==k){ result.add(new ArrayList<Integer>(curr)); return; } for(int i=start; i<=9; i++){ curr.add(i); helper(result, curr, k, i+1, sumi); curr.remove(curr.size()1); } } 
Note the following relation in Java:
Integer is a subclass of Number and ArrayList
<pre><code> String foo = "bar"; </code></pre>

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Nilesh Barge